I can't forward the link yet, but I found a nice overview of research in neuroprotectiveness:

Lazabemide Lz vs pbo delayed need for LD therapy
Pramipexole vs levodopa Initial treatment with ppx delayed complications but LD more clinically effective -- neuroimaging showed slower DAT loss with ppx
Riluzole Riluzole vs pbo demonstrated no difference in PD progression
Ropinirole vs levodopa Initial treatment with rop delayed motor complications but LD more clinically effective -- neuroimaging showed slower DAT loss with rop
Rasagiline (TEMPO) Early vs delayed start of rasagiline suggested disease modification
Levodopa LD vs placebo suggested disease modification on clinical scales but disease worsening on neuroimaging
TCH346 TCH346 vs pbo demonstrated no difference in PD progression
CEP-1347 CEP-1347 vs pbo demonstrated no difference in PD progression
Rasagiline (ADAGIO) Early vs delayed start of rasagiline suggested disease modification in 1-mg/day dose but not in 2-mg/day